NdFeB (neodymium, iron, and boron) system sintered magnets were discovered in 1982 by Sagawa, one of the inventors of this invention, and other researchers. NdFeB system sintered magnets exhibit characteristics far better than those of conventional permanent magnets, and can be advantageously manufactured from raw materials such as Nd (a kind of rare earth element), iron, and boron, which are relatively abundant and inexpensive. Hence, NdFeB system sintered magnets are used in a variety of products, such as voice coil motors used in hard disks and other apparatus, driving motors for hybrid or electric cars, battery-assisted bicycle motors, industrial motors, high-grade speakers, headphones, and permanent magnetic resonance imaging systems.
In recent years, there has been increased anticipation for a thin (in the direction of the magnetization) NdFeB system sintered magnet which can be used at ambient temperatures of 100° C. or more. Such a magnet will mainly be used in the automobile industry, which is rapidly taking on environmental and other issues. However, NdFeB system sintered magnets have a problem in that their magnetic properties significantly deteriorate as the temperature increases, and therefore an irreversible demagnetization is likely to occur at ambient temperatures of 100° C. or more. A NdFeB system sintered magnet with a coercive force HcJ (the measured value of the magnetic field H when the magnetization intensity J is 0 as a result of decreasing the magnetic field H on the magnetization curve) equal to or greater than a pre-defined value (e.g. 15 kOe≈1.2 MA/m) must be manufactured to solve this problem. A magnet having a high coercive force is less likely to be demagnetized, which decreases the likelihood of irreversible demagnetization.
One way to increase the coercive force of a NdFeB system sintered magnet is to substitute RH for a portion of Nd (substitution method). Although it can increase the coercive force, the disadvantage of this method is that the residual flux density and the maximum energy product are decreased.
Patent Document 1 discloses a method for manufacturing a NdFeB system sintered magnet using a grain boundary diffusion method. In this method, the crystal axis of each grain in a NdFeB system alloy powder is oriented in a predetermined direction. The NdFeB system alloy powder is then sintered at a predetermined sintering temperature to prepare a sintered compact, to the surface of which is applied a powder of RH or a powder of a compound of RH (which will hereinafter be referred to as an “RH powder”), and the sintered body is heated to the temperature at which RH diffuses. Naturally, this diffusion temperature is lower than the sintering temperature. As a consequence, RH penetrates into the sintered compact through the grain boundaries of the Nd2Fe14B crystal grains which exist in the sintered compact, so that RH is diffused on the surface of the crystal grains. It is possible to obtain a high coercive force and suppress the reduction in residual flux density and maximum energy product using the grain boundary diffusion method. In addition, the manufacturing cost of a sintered magnet decreases because RH, which is rare metal, is used less in this method than in the substitution method.